Ni Dots Research Articles

Transmission electron microscopy observation of a single Ni dot fabricated using scanning tunneling microscopy

Recent developments in high-technology experimental equipment and semiconductor micro fabrication technology have made it possible to study various characteristic quantum phenomena such as quantized conductance and coulomb-blockade effects at the nano-scale level. One recent study attempted to fabricate a normal-super-normal double tunneling junction in a split-gate wire. In this study, we describe the results of transmission electron microscopy (TEM) observation of a single 100 nm wide and 200 nm high Ni dot, which had been fabricated using scanning tunneling microscopy (SEM), placed on the top of the bridge of a Si-doped GaAs epitaxial layer grown by molecular beam epitaxy using a Riber 32 apparatus. The results from the successful use of TEM to observe the results of a SEM fabrication procedure support the contention that the fabricated Ni dot became a poly-crystal that was amorphous between GaAs crystal and a surface dot fabricated by applying voltage pulse to a STM tip, covered by an oxygenated film approximately 20 nm thick.

Well‐Ordered Thin‐Film Nanopore Arrays Formed Using a Block‐Copolymer Template

Nanopores in various metallic thin films are defined using self-assembled templates from a sphere-forming block copolymer (see image). This method is versatile and can be used to form antidot arrays in a variety of metals including Ti, Pt, W, Ta, Co, and Ni, and silica, as well as Ni dot arrays.

Relevance of length scales in exchange biased submicron dots

Strong dot-size dependence of the positive exchange bias onset with the cooling field was found in Ni/FeF2 exchange biased nanostructures. With increasing cooling field, the sign of the exchange bias field changes from negative to coexistence of positive and negative, and eventually to positive. As the structure size decreases, the lower limit of cooling fields necessary for only positive exchange bias also decreases and is one order of magnitude smaller than that of unpatterned films. This behavior is attributed to comparable Ni dot size with the antiferromagnet “domain” size estimated to be about 500 nm.

Magnetization anisotropy of Ni dots with several tens of nanometer diameter

We have studied the magnetization of Ni dot with 50 to 70 nanometer diameter and 12 nanometer thickness using a magnetic force microscopy with an in-plane magnetic field. The Ni dots were prepared using self-assembled dot patterns with poly (styrene-b-methyl mathacrylate) diblock copolymers on Ni film and ion etching. It was found that the remanent magnetization direction of the dot was perpendicular to the plane as prepared. From the vibrating sample magnetometer measurement, a hysteresis loop was found in the perpendicular magnetization. When an in-plane external magnetic field was applied, the magnetization was rotated into a horizontal direction with low coercivity along the field direction.

Fabrication and characterization of low temperature polycrystalline silicon thin film transistors by ink-jet printed nickel-mediated lateral crystallization

High-performance polycrystalline silicon thin film transistors (poly-Si TFTs) were fabricated by using ink-jet printed nickel on large area glass substrates. Direct-printed Ni dots were used as a catalyst to crystallize the amorphous silicon into poly-Si for use in the channel region of TFT devices by metal-induced lateral crystallization. The fabricated poly-Si TFTs showed high field-effect mobility and on-off ratio that are comparable to the TFTs prepared using conventional semiconductor processes. The method presented here is a combination of bottom-up and top-down approaches and has a potential to be used in next generation high-performance poly-Si TFT fabrication.

Applications of SU-8 in the development of a Single Ion Hit Facility

The Lund Nuclear Microprobe (LNM) has been adapted to be used as a Single Ion Hit Facility (SIHF) for proton cell irradiation experiments at low dose. In order to test the capabilities of the system, Ni dot arrays and artificial cells have been fabricated with the photopolymer SU-8 and common lithographic techniques. The primary purpose of the Ni dot arrays was to determine the targeting accuracy of the beam in vacuum and in air. Additionally, this sample was employed to evaluate the system performance during cell target irradiation experiments. The Ni dot arrays were also used for beam characterization. The artificial cells were originally fabricated to test the software for cell recognition and localization, developed and implemented at the LNM. However, this sample became very functional to make small adjustments at the irradiation chamber for cell irradiation experiments. A description of the samples, fabrication procedure and applications are presented in this paper.

Wafer-scale fabrication of patterned carbon nanofiber nanoelectrode arrays: A route for development of multiplexed, ultrasensitive disposable biosensors

One of the major limitations in the development of ultrasensitive electrochemical biosensors based on one-dimensional nanostructures is the difficulty involved with reliably fabricating nanoelectrode arrays (NEAs). In this work, we describe a simple, robust and scalable wafer-scale fabrication method to produce multiplexed biosensors. Each sensor chip consists of nine individually addressable arrays that uses electron beam patterned vertically aligned carbon nanofibers (VACNFs) as the sensing element. To ensure nanoelectrode behavior with higher sensitivity, VACNFs were precisely grown on 100 nm Ni dots with 1 μm spacing on each micro pad. Pretreatments by the combination of soaking in 1.0 M HNO 3 and electrochemical etching in 1.0 M NaOH dramatically improved the electrode performance, indicated by the decrease of redox peak separation in cyclic voltammogram (Δ E p) to ∼100 mV and an approximately 200% increase in steady-state currents. The electrochemical detection of the hybridization of DNA targets from E. coli O157:H7 onto oligonucleotide probes were successfully demonstrated. The 9 arrays within the chip were divided into three groups with triplicate sensors for positive control, negative control and specific hybridization. The proposed method has the potential to be scaled up to N × N arrays with N up to 10, which is ideal for detecting a myriad of organisms. In addition, such sensors can be used as a generic platform for many electroanalysis applications.

Enhancing Superconducting Parameters by Array of External Magnetic Dots

AbstractEnhancing the pinning force in high-Tc superconductors can be achieved by externally introduced periodic magnetic dots. We use a novel nano-technique to deposit periodic sub-micon Ni dots on YBa2Cu3O7 thin films. The current versus voltage characteristics of an YBa2Cu3O7 thin film strip with uniform Ni dots are measured at various temperatures and magnetic fields. They are compared with the current versus voltage characteristics of a bare YBa2Cu3O7 thin film strip without magnetic dots. It is found the critical current value of the strip with Ni dots reduces with a much slower pace as the magnetic field strength increases in comparison with the value of the bare sample.

Interaction between the Ferromagnetic Dots and Vortices: Numerical Calculation and Experimental Results

Enhancing the pinning force in high-Tc superconductors can be achieved by externally introduced periodic magnetic dots. We numerically calculate the interaction between ferromagnetic dots and vortices in high-Tc superconductors. The London equation is used to generate two-dimensional vortex lattice. In the matching condition, we calculate the attraction force between magnetic dots and vortices. It is found that in an ideal condition, the pinning force of the magnetic dot reaches 2.5×10−11 N that is more than one order magnitude stronger than the intrinsic pinning force in YBa2Cu3O7 thin films. In the experimental side, we use a novel nano-technique to deposit periodic submicron Ni dots on YBa2Cu3O7 thin films. The current versus voltage characteristics of an YBa2Cu3O7 thin film strip with uniform Ni dots are measured at various temperatures and magnetic fields. They are compared with the current versus voltage characteristics of a bare YBa2Cu3O7 thin film strip without magnetic dots. It is found the critical current value of the strip with Ni dots reduces with a much slower pace as the magnetic field strength increases in comparison with the value of the bare sample.

Flame-synthesis of carbon nanotubes on silicon substrates and their field emission properties

We reported the flame-synthesis of patterned multiwalled carbon nanotubes (CNTs) on silicon substrate by a shadow mask and their field emission properties. It was found that CNTs with tangled and curved morphology were preferentially grown around the cracked edges of Ni dot pattern. A crack-induced catalyst-activation growth mechanism was proposed. The patterned CNTs fabricated by such a simple flame-synthesis method exhibited good field emission characteristics with uniform emission patterns and reproducible and stable emission behaviors, although the CNTs possessed many defective graphite layers and showed relatively higher turn-on and threshold field than other reported CNTs grown by chemical vapor deposition. Our results demonstrated that such a low-cost and scaleable CNT pattern fabrication process can be expected to have favorable applications in field emission devices.

Self-Assembling Formation of Ni Nanodots on SiO2 Induced by Remote H2 Plasma Treatment and Their Electrical Charging Characteristics

We fabricated nanometer-scale Ni dots and NiSi dots on an ultrathin SiO2 layer using remote H2 plasma and demonstrated the feasibility of remote H2 plasma treatment for controlling the areal density of the dots. 1.8-nm-thick-Ni/SiO2 and Ni/Si-quantum dots (QDs)/SiO2 layer were treated with remote H2 plasma generated by the inductive coupling between an external single-turn antenna and a 60 MHz generator. When a Ni/SiO2 was treated with remote H2 plasma at room temperature, Ni nanodot density could be controlled in the range of 109 to 1012 cm-2 by adjusting the plasma conditions. After the remote H2 plasma treatment of the Ni/Si-QDs, the formation of electrically isolated NiSi dots with an areal density of ∼1011 cm-2 was confirmed. These results imply that hydrogen radicals generated in H2 plasma play an important role in improving surface diffusion caused by energy reduction at the Ni/SiO2 interface. The surface potential of the Ni nanodots changes stepwise with the tip bias. This is due to the multistep electron injection into and extraction of Ni nanodots. The minimum tip biases for electron injection into Ni nanodots, NiSi dots and Si-QDs were -0.2, -0.7, and -1.0 V, respectively. This reflected the difference in electron affinity among Ni, NiSi and Si.

Periodic arrays of submicron Si and Ni dots on SiO2 fabricated using linearly polarized Nd:YAG pulsed laser

Periodic arrays of submicron Si and Ni dots were fabricated by only irradiating a linearly polarized Nd:YAG pulsed laser beam to Si and Ni thin films deposited on silicon dioxide (SiO2) film. The interference between an incident beam and a scattered surface wave leads to the spatial periodicity of beam energy density distribution on the surface of the irradiated samples. A thin film was melted using a laser beam, and the molten film was split and condensed owing to its surface tensile according to the periodic energy density distribution. Then, the fine lines (line and space structure) were formed periodically. After the formation of fine lines, the sample was rotated by 90°, and the laser beam was irradiated. The periodic energy density distribution was generated on the fine lines, and the lines were split and condensed. Eventually, the periodically aligned submicron dots were fabricated on the SiO2 film.

Characterization of Electronic Charged States of Nickel Silicide Nanodots Using AFM/ Kelvin Probe Technique

We have formed high density nanodots of nickel silicide (NiSi) on ultrathin SiO2 and characterized their electronic charged states by using an AFM/Kelvin probe technique. Si quantum dots (Si-QDs) with an areal dot density of ~2.5x1011cm-2 were self-assembled on ~3.6nm-thick thermally-grown SiO2 by controlling the early stages of LPCVD using pure SiH4 gas. Subsequently, electron beam evaporation of Ni was carried out as thin as ~1.7nm in equivalent thickness at room temperature and followed by 300°C anneal for 5min in vacuum. XPS and AFM measurements confirm the formation of NiSi dots with an average dot height of ~8nm. After removal of Ni residue on SiO2 by a dilute HCl solution, bias conditions required for electron charging to NiSi dots were compared with those to pure Si-QDs dots and Ni dots. The surface potential changes stepwise with respect to the tip bias due to multistep electron injection and extraction of NiSi nanodots. In addition, it is confirmed that charge retention characteristics of NiSi dots are superior to those of Si-QDs with the almost same size.

Quantitative analysis of the guidance of vortices in superconducting films with magnetic dots

The guidance of vortices in superconducting Nb films with periodic arrangements of magnetic Ni dots is studied via simultaneous four-probe measurements in standard and Hall configuration. Ni dots are fabricated via e-beam lithography and lift-off technology. Arrays with periodicity down to 400 nm and dot size of 200 nm are obtained. Subsequently, the arrays are covered with a thin superconducting Nb layer. Simultaneous Hall and standard resistive measurements of the flux dynamic in Nb bridges with and without magnetic dots demonstrate and quantify the guidance of flux via the magnetic dots. Guidance of vortices is only possible in a limited current regime; guidance improves with decreasing temperature. Within the identified current regime most of the vortices (>40% for zero field) and in the optimum case all of the vortices are guided by the magnetic dots.

Interfacial reactions of 2-D periodic arrays of Ni metal dots on (0 0 1) Si

The fabrication of 2-D periodic arrays of nickel metal dots using polystyrene (PS) nanosphere lithography and the interfacial reactions of the Ni dot arrays on (0 0 1) Si substrates after different heat treatments have been investigated. The epitaxial NiSi 2 was detected to start growing in samples after annealing at 300 °C. As the annealing temperature was increased to 350–800 °C, only epitaxial NiSi 2 nanodot arrays were observed to form. The results revealed that the formation of epitaxial NiSi 2 is more favorable for the Ni metal dot arrays samples. The shape of the epitaxial NiSi 2 nanodot was found to be inverse pyramidal and the size of the silicide nanodots was measured to diminish with annealing temperature. Furthermore, amorphous SiO x nanowire arrays and single-crystalline Si nanowires were found to form in samples after annealing at 900 and 1100 °C, respectively. The results present the exciting prospect that other nanoscale metal silicide dot and nanowire arrays could be grown on the sub-100 nm pre-patterned Si substrates using the PS nanosphere lithography technique.

Fabrication of Periodic Arrays of Nano-sized Si and Ni dots on SiO2 Using Linearly Polarized Nd:YAG Pulsed Laser

ABSTRACTPeriodic arrays of nano-sized Si and Ni dots were fabricated by only irradiating a linearly polarized Nd:YAG pulsed laser beam to Si and Ni thin films deposited on silicon dioxide (SiO2) film. The interference between an incident beam and a scattered surface wave leads to the spatial periodicity of beam energy density distribution on the surface of the irradiated samples. A thin film was melted using a laser beam, and the molten film was split and condensed owing to its surface tensile according to the periodic energy density distribution. Then, the fine lines (line and space structure) were formed periodically. After the formation of fine lines, the sample was rotated by 90°, and the laser beam was irradiated. The periodic energy density distribution was generated on the fine lines, and the lines split and condensed according to the periodic energy density distribution. Eventually, the periodically aligned nano-sized dots were fabricated on the SiO2 film.

Nanosoldering of carbon nanotubes on metal electrodes with an atomic force microscope

Contacts of carbon nanotubes to the prefabricated metal electrodes are improved by field evaporation of metals from a tip of an atomic force microscope such as soldering and/or the thermal treatment. The resultant contact resistance is as good as ∼100kΩ with nanoscale Au dots on Au electrodes, similar to the result of thermal treatment of the device. The contact resistance is found to be lower with Au dots than with Ni or Al dots. This method can be applied to local soldering of nanowires and nanotubes to the metal electrodes.

Study of electron field emission from arrays of multi-walled carbon nanotubes synthesized by hot-wire dc plasma-enhanced chemical vapor deposition

Multi-walled carbon nanotubes have been grown on 7nm Ni-coated substrates consisting of 300μm thick highly n-doped (100) silicon covered with a diffusion barrier layer (10nm thick) of SiO2 or TiN, by combining hot-wire chemical vapor deposition and direct current plasma-enhanced chemical vapor deposition at low temperature (around 620°C). Acetylene gas was used as carbon source and ammonia and hydrogen were used either for dilution or etching. Growth of dense aligned nanotubes could be observed only if the ammonia content was minimized (∼5%). In order to improve the electron field emission properties of the films, different geometrical factors have been taken into account: average length, length/radius ratio and spacing between nanotubes. The nanotube growth rate was controlled by the substrate temperature and the pressure in the reactor, and the nanotube height by the growth time. The nanotube diameter was controlled by the catalyst dot volume, and the nanotube spacing was adjusted during the patterning process of the catalyst dots. Using optical lithography, 1μm Ni dots were obtained and several multi-walled nanotubes with diameter and length in the range 60–120nm and ∼2.3μm were grown on each dot. Thus, based on a two-dimensional square lattice with a lattice translation vector of 4μm, I–V characteristics yielded an onset electric field of 16V/μm and a maximum emission current density of 40mA/cm2, due to the large screening effect. Using electron-beam lithography, 100nm Ni dots were obtained and individual multi-walled nanotubes were grown on each dot. Based on a square lattice with 10μm translation vector, I–V characteristics gave an onset field of 8V/μm and a maximum emission current density of 0.4A/cm2.

Magnetic field induced suppression of vortex flow resistance in superconductors with periodic pinning centers

We study vortex flow resistance (VFR) in films of Pb with holes as periodic pinning centers (PPCs) and of Nb with PPCs in the form of Ni dots, as function of temperature, dc current and constant applied magnetic field. The experimental resolution is better than 10−5 of the normal state resistance. At high temperatures near to Tc and high drive currents the resistance shows local minima both at matching fields and zero field. For lower temperatures, however, in a narrow temperature range before the vortex system becomes completely frozen, we observe suppression of the VFR with increasing magnetic field. In the Pb film with PPCs these phenomena show up as a clear zero field resistance excess, which is gradually suppressed by the applied magnetic field. We attribute this unusual feature in the magnetoresistance to thermally excited vortex–antivortex pairs. In the Nb superconducting film with PPCs we observe a gradual suppression of the VFR near integer matching fields up to n=3 at the lowest accessible dc currents and relevant temperatures. This anomalous behaviour is followed by the observation of negligible VFL for higher fields (H3<H<H4) and a strong enhancement of VFR for H>H4. We tentatively explain this observation as due to the moving vortex glass.

Tribological behavior of nano-undulated surface of diamond-like carbon films

Tribological behavior of nano-undulated diamond-like carbon (DLC) films of the surface roughness ranging from 0.6 to 13.7 nm was investigated in an ambient air of 50% relative humidity. The nano-undulated DLC films were prepared by radio frequency plasma-assisted chemical vapor deposition (r.f.-PACVD) using nanosized Ni dots on a Si (100) substrate. The friction coefficient between the DLC film and the steel ball was characterized by a ball-on-disk type wear rig. Auger and Raman spectroscopy analysis of the debris revealed that the tribochemical reaction with environment was significantly suppressed as the surface roughness increased. Even if the rough surface increased the wear rate of the steel ball and thus the concentration of Fe in the debris, neither the oxidation of Fe nor the graphitization of the carbon in the debris occurred on the rough surface. However, the frictional behavior was affected by several factors: the composition and the size of debris, plowing effect of the rough surface, and the presence of the transfer layer on the wear scar surface.